Health and welfare POA is a pretty grey area, get one if you can but the medics can always override it as they have to work in the patient's best interests.
It does give you access to some medical appointments and decisions but it isn't the be all and end all when decisions have to be made and the person with the LPA is always an outsider in the patient-doctor relationship.
The medics won't make some decisions without involving the family and where the relationship is clear cut like husband/wife it's not a problem but after that and a son/daughter then are there other siblings, why should one be asked when there may be many who should be consulted and when their relationship becomes more important than all the rest unless they have an LPA?
Many people on here deal with in laws or aunts/uncles where the relationship and how many other people may be involved at the same level of familiarity is unclear, there a POA is very much needed.
If you can get an LPA as I assume the joint LPA you have with your sister is for finance so it isn't clear, who has the final say on the important issues of health and welfare?
K